High voltage resistor



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United States Patent 'O mon VOLTAGE Rasisron Robert C. Langford, WestOrange, NJ., assignor to Daystrom, Incorporated, Murray Hill, NJ., acorporation of New Jersey Filed Jan. 19, 1959, Ser. No. 787,602

8 Claims. (Cl. SSS-260) This invention relates to a resistor and moreparticularly to a high voltage resistor which is capable of withstandingsevere thermal shock, without subsequent failure, which has a low`operating temperature under over- 'load conditions, and which has auniform voltage gradient distribution along the resistor.

The high voltage resistor contemplated by my invention includes aplurality of resistor elements connected together in series circuitarrangement and embodied in a unitary structure including, generally, atubular body member of insulating material within which the resistanceelements are suitably mounted, and metallic end caps closing the ends ofthe tubular body member and providing electrical terminals for theresistor. It has been found, however, that some separation invariablytakes place between the insulating tube and the metallic end caps whenthe resistor is subjected to thermal shock. In prior art high voltageresistor construction, it is then possible for moisture to enter thetube with deleteriousv effects on performance, accuracy and life. In thehigh voltage resistor construction of my invention, the resistor tubehaving plastic end caps thereon. The metallic end caps are positionedlover the said plastic end caps. The plastic mounting strip, tube andend caps are all made of the same plastic material h-aving the sametemperature coellicient of expansion. Therefore, even if the metallicend caps separate from the plastic tube, the tube remains sealed by theplastic end caps, and moisture cannot enter therein. The mounting of theresistance elements on a plastic mounting strip, and the encapulationthereof in a plastic tube with sealed plastic end caps, in which all ofthe above components are of the same plastic material, comprises animportant aspect of my invention.

During overload current conditions, the excessive temperature rise byheat generated in high voltage resistors of prior art constructionseither results incomplete failure of the resistor, 'or results inexcessive change in resistance of the resistor due to the temperaturecoefficient, for elective failure thereof. In the resistor of myinvention, the

being asphalt, for example.

solid. As the temperature rises under'overload condi,- tions, the excessheat developed is absorbed in the heat of fusion of the lilling materialas the material turns from a solid to a liquid. The temperature insidethe tube Will therefore remain at the melting point of the iillingmaterial until the whole of such material is melted. This provides amuch lower operating temperature under overload conditions which, inturn, with a usual temperature coefficient of resistance, results in asmaller change in the resistance thereof. The filling of the tubularhousing, within which the resistor elements are mounted, with aninsulating material which is solid under normalele' vated ambientworking conditions, and which melts under overload conditions, comprisesan important feature of my invention.

Another important feature of my invention involves the positioning ofthe resistor elements on the insulated mounting strip. The elongatedresistance '-elements are laid out in a parallel extending, uniformallyspaced, arrangement whereby the stray capacitance of each resistanceelement to ground is identical. Further, the metallic end caps closingthe ends of the tubular body member are positioned as close togetheras'possible to make the shunt capacitance therebetween large, and theseries capacitance between each resistor element small. Preferably, theshunt capacitance is about ten (l0) times as great as the said seriescapacitance. This results in a more nearly uniform voltage gradientdistribution along the resistor, which is of considerable importance inhigh voltage resistor construction, used under alternating voltageconditions.

An object of this invention is the provision of a high voltage tubularresistor which is able to withstand severe thermal shock and towithstand overload conditions following a full load run with a minimumof deleterious elects.

An `object of this invention is the provision of a high voltage resistorincluding a plurality of resistance elements connected in series circuitarrangement and each having substantially identical stray capacitancefrom the ends thereof to ground.

An-olbject of this invention is the provision of a hig voltage resistorconstruction which includes a plurality `of resistor elements mounted ona strip of plastic insulatelements are mounted on a plastic strip withina plastic ing material, a tubular member of the same insulating materialwithin which the mounting strip is mounted, end

caps of the same plastic material closing the ends of the tube, andmetallic lend caps mounted over the plastic ends and electricallyconnected to the resistor elements.

An object of this invention is the provision of a high voltage resistorconstruction comprising a container, a resistance element embedded in aninsulating material. which is solid under normal ambient working tempera-ture Within the said container, the said insulating matesistanceelements positioned in an evenly spaced, parallel, l, position within acontainer of insulating material whereby the stray capacitance from suchresistance element end to ground is substantially identical.

These and other objects and advantages will become` apparent from thefollowing description when taken with It will be understood that theaccompanying drawings. the drawings are for purposes of illustration andare not intended to be construed as defining the scope or limits of theinvention, reference being had for the latter purpose to the appendedclaims.

In the drawings, wherein like reference characters l refer to like partsin the several views:

" for clarity;

Figure 2 is a cross sectional view of the resistor taken on line 2-2 ofFigure l;

Figure 3 is a cross sectional view of the along the line 3-3 of Figure1;

Figure 4 is ga fragmentary view of one end of. the'- resistor showingthe plastic which forms'the end cap in' position in the tube prior toturning the metalliczendzl.;

cap onto the tube end; f Figure 5 is a fragmentary view which' isresistor 'taken' similar to 3 Figure 4 only showing the metallic end capturned onto the tube;

Figure 6 is a chart of temperature versus time for resistor operationunder normal and overload conditions; and

Figure 7 is a fragmentary view of the resistor element mounting stripand schematically showingr in.l broken lines stray capacitance existingthereon.

Reference is rst made to Figures l', 2. and 3 of. the drawings whereinAthe resistor of my invention is shown comprising a tubular housing 1t)of insulating material, preferably epoxy resin, having internallythreaded metallic end caps 11,v i1 tbreadedly attached to the tube ends.External, annular flanges 12, 12 are formed on the end caps for mountingthe high voltage resistor by any suitable means notY shown in thedrawings. A plurality of resistor elements: 13 are attached to amounting strip 14 of insulating material, within the tubular housing 10,by any suitable means not shown.

The resistor elements 13 are not limited to any particular type, andtherefore, are not shown in detail in the drawings. Resistor elements ofthe wire wound or film type, for example, may be employed. For highvoltage use, however, film type resistor elements are preferablyutilized in which the said elements each comprise a/ cylindrical ceramictube 16 with a resistive film disposed on the internal surface thereof.Preferably, the said film is an alloy of the nickel-chromium family,which is deposited on a glaze on the internal surface of the saidceramic tube, which is then fired at a high temperatureV therebydispersing theA resistance film throughout the body of the glaze.Adjustment of the resistance of the dispersedv lrn ismade in aconventional manner by cutting a groove in the form of an internal helixtherein until the desired value of resistance is obtained. Metallicresistor. element end caps 17, 17 and leads are pressed over thev endsyof the cylindrical tube 16 and suitably connected to the resistive lilmwithin the tube. Although not shown in the drawings, each resistorelement may be encased in a suitable plastic shell of epoxy resin, ortheV like. Resistor elements of this construction exhibit good stabilityand good high frequency performance, and are therefore desirable for usein the construction of a precision, high voltage, resistor made inaccordance with my invention. The maximum voltage stress in resistorelements of the type wherein a resistance film is deposited on theinternal surface of the ceramic tube, obtains in the said ceramic tube,which tube may be made of closely compacted material which is void ofair pockets. Such resistance elements are capable of withstanding highvoltages without breakdown. Resistance elements, which include aresistive film, or the like, which film is merely coated or covered byan insulating material, often includes air pockets between the coatingand film. Resistance elements of this type are not suited for highvoltage use since the air in the pockets ionizes under high voltagestress, and nitric acid is thereby formed which destroys the resistivefilm.

Asbest seen in Figure l, the resistor elements are connected together ina series circuit arrangement. The end resistor elements are connectedthrough lead wires 18 to the metallic end caps 11, the wires 18extending through a suitable hole in the end caps andsoldered to theends of the capsv as. at 19. (The connection to only one end cap 11 isshown in the drawing.)

'Plastic end caps, or sealing members, 21, 21 of insulating materialclose the ends of the tubular housing 10. The plastic end caps areVprovided with integrally formed radial flanges 21a, 21a positionedbetween the ends of the tubular housing and the metallic end caps11,.Which flanges are formed during the construction of the resistor ina manner described hereinbelow. The ends of the mounting strip 14 areembedded in the cylindrical. body portion ofthe said plastic end caps,or sealing members 21, 21, for support Within the tubular housing 10.

In accordance with my invention, the tubular housing 10, mounting strip14 and plastic end caps, or sealing members 21, 21 are made of materialhaving the same temperature coefficient of expansion. To this end, theabove components are made of the same kind of plastic material and,preferably, the material comprises an epoxy resin. Although otherinsulating material may be used, epoxy resinhas at least two propertiesmaking it particularly well suited for use therein. ln the first place,epoxy resin, when polymerized during the manufacture of the resistor,does not. release water, or water vapor, `which would be highlydeleterious in high voltage use. Secondly, the epoxy resin materials,upon polymerizing, form a firm bond with everything with which they arein contact, including other epoxy resin material. Therefore, by makingthe tubular housing, sealing members and mounting strip of epoxy resinmaterial having the same temperature coefficient of expansion, noseparation of these elements results under thermal shock conditions.With my construction, even if the metallic end caps 11, 11 do separatefrom the tube, moisture, and the like, cannot enter the tubular housingsince it remains sealed at the ends.

All resistance elements have a temperature coefcient of resistance; thatis, the resistance thereof changes with temperature. Although thetemperature coefficient may be either positive or negative, mostresistance elements display a positive temperature coefcient. However,regardless of whether the temperature coecient of the resistor elementsin the resistor is positive or negative, it will be understood that tofunction as a precision resistor large resistance changes under variousload conditions cannot be tolerated. In the resistor of my inventionnovel means are provided whereby, during limited periods of overloadconditions, the temperature of the resistance elements is prevented fromrising above a predetermined value. This, in turn, limits the change inresistance due to the usual temperature coeiiicient. In accordance withmy invention, the tubular housing between the insulating end caps 21, 21is filled with a filler 22 of insulating material which is solid undernormal ambient working temperatures, and which melts under overloadconditions. Such a filler may comprise, for example, asphalt.

Before describing the operation of the resistor under normaland overloadconditions, reference is rst made to Figures 4 and 5 of the drawingswherein successive steps in the assembly ofv end caps to one end of thetubular housing 10 by one. method of assembly are shown. The mountingstrip 14, with the resistor elements 13 attached thereto, is positionedin the tubular housing 10 and a mass of epoxy resin material, designated2lb, is placed in the tubular housing end, with a rounded portion of thematerial extending beyond the housing end. The epoxy, resin 2lb ispreferably in a pliable, semisolid, condition which is easily handledand worked, and comprises the plastic end cap 21 after metallic end cap11 is turned onto the tubular housing 19, as shown in Figure 5. As themetallic end cap 11 is turned onto the threaded tubular housing end, theepoxy resin mass is shaped to conform to the surrounding structurewhereby the flanges 21a are formed between the tubular housing endandthe metallic end cap.

The partially assembled resistor is then placed in a vertical positionwith the open. end of the tubular housing at the. top, and the liquidasphalt 22 is poured therein to the desired level. Plastic and metallicend caps are then placed over the upper free end of the tubular housingin substantially the same manner as the caps at the other end, asdescribed above. It will be noted that the ends' of the mounting strip14 are embedded in, and supportedby, the plastic end caps, or sealingmembers 21,

21. After polymerization, the tube 10, end caps 21 and the ends ofmounting strip 14 are bonded together into a substantially unitarystructure of like material.

As mentioned above, the asphalt filler 22 is solid under normal ambientworking temperature. If, then, for eX- ample, a normal ambient Workingtemperature of 85 C. obtains when the resistor is dissipating fullwattage, the ller remains solid if its melting point thereof is above 85C. A graph of the plot of temperature versus time for both normal andoverload conditions is shown in Figure 6, thc normal load conditionbeing illustrated in solid line and the overload condition in brokenline. Under normal full load condition, it will be noted that theresistor reaches an ambient temperature of Temp. 1, which, in the aboveexample, may be 85 C. The melting point of the filler 22 is Temp. 2,which is above the normal operating temperature, and may be, forexample, 100 C. Thus, under normal operating conditions, the lillerremains solid. If, however, at time T1 during normal load conditions, asubstantial overload is placed on the resistor, the temperature willproceed to rise until the melting point of the filler is reached. Whenthe melting temperature of the ller is reached, at time T2 on the graph,energy is dissipated in the filler liquid heat of fusion as the fillerturns from a solid to a liquid. Thus, the temperature of the resistorwill remain at the melting point of the ller until the whole of theliller is melted. When all of the ller is melted at time T3 on thegraph, the temperature will again rise. Between time T2 and T3, underoverload conditions the temperature remains constant. Only a relativelysmall temperature rise is encountered between the normal operatingtemperature and the melting point of the filler, between times T1 andT2. With a usual temperature coefcient of resistance, the resistancewill change only a relatively small amount during brief overloadconditions since the temperature rise is small. The use of such fillermaterial is, as mentioned above, of particular value in precisionresistors where only small resistance changes may be tolerated.

Film type resistance elements of the type preferably used in theconstruction of the resistor of my invention are particularly adaptedfor high frequency use since the capacitance thereof is small ascompared with wire wound resistors, for example. With good qualityprecision resistance elements 13, the only reactive component of anyconsequence is due to distributed capacitance between the metallicresistance element end caps 17, 17, which is on the order of severalmicromicrofarads. Where a plurality of resistance elements are utilizedin the construction of precision high voltage resistors, not only mustthe capacitance of the individual elements be small for high frequencyuse, but substantially identical capacitance between the ends of theresistors in the assembly thereof is necessary so that the inevitablestray capacitance of each resistor end to ground will be the same. Thisresults in a more uniform voltage gradient distribution, necessary forhigh voltage use.

Reference is made to Figure 7 of the drawings wherein there is shown afragmentary view of the mounting strip 14 with resistors 13 attachedthereto. The resistors are arranged in a uniform manner on the stripwith identical stray capacitances 26 (shown in broken lines) between theleads from each resistor element. The capacitance between resistanceelement end caps 17, 17 is designated 27. By using all of the same typeresistor elements whereby the capacitance 27 for each resistor isidentical, and by positioning the resistors on the board 14 in a mannerwherein the stray capacitance 26 between the ends of each of theindividual resistors is identical, the inevitable stray capacitance toground from each lead of the resistors (designated 28) is alsosubstantially identical. For high voltage, alternating current use, asubstantially uniform voltage gradient distribution along the resistoris obtained.

The stray capacitance 26 by the ends of the resistor elements are madeidentical by positioning the resistor elements in an aligned anduniformly spaced relation, within a single plane. In the illustratedarrangement, this is accomplished by mounting the resistance elements atan acute angle with a straight line, designated 31, through the alignedresistors. Obviously, the resistor elements may be placed coaxiallywith, perpendicular to, or at any desired angle with the line 31, solong as they are located in a common plane and are uniformly spaced toprovide substantially equal stray capacitances 26 between ends.

Having now described my invention in detail, in accordance with thepatent statutes, various changes and modications will suggest themselvesto those skilled in this art, and it is intended that such changes andmodifications shall fall within the spirit and scope ofthe invention asrecited in the following claims.

I claim:

1. A resistor `structure comprising `a tubular member, end caps closingthe ends of the tubular member, an elongated mounting strip disposed inthe tubular member and having ends embedded in the said end caps, andresistor elements mounted on the mounting strip, the tubular member, endcaps and mounting strip being made of insulating material having thesame coefficient of expansion.

2. The invention as recited in claim 1, wherein the said tubular member,end caps and mounting strip yare made of epoxy resin.

3. The invention as recited in claim l, including genera'lly cup-shapedmetallic end caps having internally Ithreaded portions threaded to theends of the said tubular member and covering thc said end ca-ps ofinsulating material.

4. A resistor structure comprising a tubular member of insulatingmaterial, end caps of insula-ting material olosing the ends of thetubular member, a mounting strip of insulating material, and a pluralityof series connected resistors attached to the mounting yst-rip, the saidmounting strip being positioned within the tubular member with the endsthereof embedded in the said end caps.

5. The invention as recited in claim 4 wherein the said tubular member,end caps and mounting strip are made of epoxy resin.

6. A resistor comprising a container, lling material within thecontainer, and a resistance element embedded in the filling material,the said lling material being solid under full load operating conditionsof the resistor and liquid under predetermined overload operatingconditions, the temperature of the resistance element remainingsubstantially constant under overload operating conditions between thetime the filling material begins to liquify and is completely liquited.

7. The invention as recited in claim 6 wherein the said lling materialcomprises asphalt.

8. The invention as recited in claim 7 wherein the said containercomprises a tubular housing of insulating material closed at both endsby end caps of insulating ma.- terial.

References Cited in the le of this patent UNITED STATES PATENTS2,030,460 Morton Feb. 11, 1936 2,466,211 Crockett -a Apr. 5, 19492,862,088 Mairs Nov. 25, 1958

